sometimes the rpms on my truck start going up and down when driving at about 50...

J

JonR

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...mph.whats going on? this only happens once in a while, but sometimes when coasting along at about the 45 to 55 mph range the rpm odometer will start rising and falling about 300 rpms.you can feel it drive all jumpy like youre rocking back and forth like its engaging and stopping erratically .it is an automatic 1998 gmc sonoma.whats going on?have you encountered this?is my tranny totally messed up?
 
Are you going uphill or downhill when this happens?

It is possible that your automatic transmission is changing gears. It is also possible that you have you cruise control on.

Check the fluid level in your transmission.
 
Are you going uphill or downhill when this happens?

It is possible that your automatic transmission is changing gears. It is also possible that you have you cruise control on.

Check the fluid level in your transmission.
 
Is your Air Conditioning on when its doing this?

A lot of times your engine will rpm's will kick up when the compressor engages and then disengages.... That would also make it kinda lunge a little bit at a light when it engages.......
Usually this will mean that your compressors bearings are going out....

(only reason i know this, is because that sounds like the same exact thing my little S10 did and that was the problem)

So turn off your A/C and see if it does it. Remember that if you have it on Defrost your A/C is running........
 
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